1. Field of the Invention
The present invention relates to a reflowing apparatus and a reflowing method for mounting parts on a printed wiring board.
2. Description of the Related Art
Recently, it is a worldwide trend to abandon the use of harmful substances from the production of electronic products in a drive to keep up with the global environment, and it is becoming essential to use a “lead-free solder” instead of using “lead-containing solder” for joining electronic products to the printed wiring boards.
The lead-free solder, however, must be treated at a temperature higher than the temperatures for the ordinary solders. Concretely speaking, the conventional eutectic solder has a melting point of 183° C. in the case of, for example, the lead-containing solder whereas the lead-free solder has a melting point of about 206° C. in the case of the “tin-silver-bismuth-indium (Sn—Ag—Bi—In)” solder and a melting point of about 220° C. in the case of the “tin-silver-copper (Sn—Ag—Cu)” solder. Besides, the solder tends to exhibit poor wet-spreading property and, therefore, the parts and materials must have higher heat resistance so will not to exhibit deteriorated properties.
As the electronic products are becoming highly functional in recent years, further, differences are increasing in the soldering temperature depending upon the kinds and sizes of parts mounted on the printed wiring board and upon the degree of packaging density. In particular, the power source circuit employs parts many of which having large heat capacities, such as ferrite coils, and the temperature does not increase so much at the junction portions. In the LSIs and memories having small heat resistance, on the other hand, parts have been fabricated in ever small sizes and in ever small thicknesses, and the temperature of parts increases at the time of soldering. Thus, the problem is becoming serious concerning an increase in the difference of temperatures and out of balance of temperatures.
Effort has now been made to produce parts which are highly heat resistant but it is difficult to provide parts which satisfy differences in the reflowing temperature under all conditions. Therefore, it has been strongly urged to provide means for solving the difference of temperature at the time of reflowing from the standpoint of producing all electronic products without using lead.
FIG. 1 is a view schematically illustrating a conventional reflowing apparatus, wherein reference numeral 101 denotes a reflowing apparatus, 102 denotes printed wiring boards, 111 and 112 denote heaters, 111a and 112a denote hot airs, and 113 denotes a board conveyer.
Referring to FIG. 1, the conventional reflowing apparatus 101 includes hot air heaters/infrared ray heaters 111 and 112 for heating provided over and under the printed wiring boards 101 that are conveyed by the board conveyer 113, wherein the hot airs 111a and 112a are blown onto the printed wiring boards 102; i.e., the printed wiring boards 102 are heated under the uniform conditions by controlling the temperatures of the heaters 111 and 112.
Here, the individual printed wiring boards 102 are heated under different heating conditions depending upon the density of the parts, specific heat, coefficient of heat conduction, absorbency for infrared rays and arrangement of the mounted parts. That is, if the printed wiring boards 102 are heated under the uniform heating condition, difference in the temperature occurs among the parts, and between the soldered portions and the parts. Further, when the lead-free solder is used as described above, the temperature difference becomes more conspicuous due to an increase in the junction temperature casting a barrier against the effort for abandoning the use of lead.
To cope with this problem, the soldering treatment has, so far, been manually carried out separately from the reflowing work (reflow soldering work) for the power source circuit that included ferrite coils having large heat capacities permitting the temperature to rise less at the junction portions. Or, the printed wiring board as a whole is heated to meet the junction portions where the temperature does not rise so much while providing a shielding plate for the portions where parts are mounted which may be so heated as to exceed their bearable temperatures in order to suppress the rise of temperature.
FIG. 2 is a view schematically illustrating a state of using the shielding plate in the reflowing apparatus of FIG. 1, wherein FIG. 2(a) illustrates a case where the shielding plate 104 is provided under the printed wiring board 102 and FIG. 2(b) illustrates a case where the shielding plate 104 is provided over the printed wiring board 102.
Here, the reflowing condition (reflow soldering work) is set to meet the soldering material that uses the minimum temperature of the junction portions, and is set to be, for example, about 230° C. in the case of the Sn—Ag—Cu solder. Usually, parts of large heat capacities such as coil in the power source system, module parts and junction portions of large connectors, are the low-temperature parts. Further, the temperature of the parts at the time of reflow soldering becomes higher than the temperature of the junction portions. Besides, parts of which the heat conditions must be controlled have bearable temperatures which are, usually, 235° C. to 245° C. and, therefore, many parts are subject to be heated beyond their bearable temperatures.
Referring to FIG. 2(a), the printed wiring board 102 is placed on a pallet 103 via a plurality of pins 131. Here, when the printed wiring board as a whole is to be heated to meet the low-temperature parts such as coils in the power source system of which the temperature does not rise so quickly, a semiconductor integrated circuit 121 is heated in excess of its bearable temperature. Therefore, the shielding plate 104 is arranged under the semiconductor integrated circuit 121 to shield the hot air from the lower side, so that the temperature of the semiconductor integrated circuit 121 is not excessively elevated.
Further, as shown in FIG. 2(b), the shielding plate 104 is stuck by using, for example, a heat resistant double-sided adhesive tape 141 onto the semiconductor integrated circuit 121 mounted on the printed wiring board 102 to shield the hot air from the upper side, so that the temperature of the semiconductor integrated circuit 121 is not excessively elevated.
As described above, when the printed wiring board as a whole is to be heated to meet the low-temperature parts such as coils in the power source system of which the temperature does not rise so quickly, the shielding plate 104 is provided for the part 121 which will be heated in excess of its bearable temperature so as to shield the hot air blown to the part 121 from the upper side thereof or from the lower side thereof to suppress the part 121 from being excessively heated.
However, when there are a plurality of parts that use the shielding plates 104, the heating efficiency greatly drops around the shielding plates 104 and the solder is not often melted. In the case of FIG. 2(b), the printed wiring board 102 warps due to an increased weight caused by the shielding plate 104, or the soldering is not perfectly effected and the quality drops.
There has further been proposed a reflowing apparatus which is capable of decreasing the temperature difference between the mounted part that can be easily heated and the mounted part that is not heated so quickly (see, for example, patent document 1).
FIG. 3 is a view schematically illustrating another conventional reflowing apparatus, i.e., illustrating the reflowing apparatus disclosed in the patent document 1.
Referring to FIG. 3, the conventional reflowing apparatus 101 includes a plurality of infrared-ray heaters 115 and 116 over and under the printed wiring boards 102 conveyed by the board conveyer 113, and block-like infrared-ray heaters 117 and 118 over and under a main heating unit HA in the furnace body for heating the printed wiring boards 102, so as to heat the printed wiring boards 102 conveyed by the board conveyer 113 by taking the rate of conveyance into consideration and in compliance with a temperature distribution pattern. The upper infrared-ray heaters 115 and the block-like infrared-ray heater 117 are provided with stirrer fans 114.
Further, the patent document 1 teaches mounting a temperature distribution pattern for decreasing the dispersion of temperature caused by the kinds and positions of parts on the printed wiring board on a heating pallet equipped with a heating unit which comprises a plurality of infrared-ray heaters, and executing the reflowing by conveying the heating pallet by the board conveyer.
Patent Document 1: JP-2002-324972-A (FIGS. 1 and 5)
As described above, there has heretofore been proposed the reflowing apparatus which effects the reflow soldering by imparting heating conditions that differ depending upon the blocks to the block-like infrared-ray heaters, and heating the printed wiring board depending upon the temperature distribution.
However, the conventional reflowing apparatus uses the infrared-ray heaters and the stirrer fans, the infrared-ray heaters having a slow speed of response. To vary the temperature conditions of the heaters, therefore, a time of several minutes is required at the shortest. Further, the heat radiated from the infrared-ray heaters interferes among the blocks making it difficult to effect local heating. It is further difficult to effect the control in synchronism with the speed of conveying the printed wiring boards, making the apparatus far from of practical use.
FIG. 4 is a view illustrating an example of a simulated temperature distribution at the time of reflow soldering in a conventional reflowing apparatus, and illustrates a temperature distribution of a printed wiring board 201 used for a notebook personal computer of when the hot air is blown to the entire board.
In FIG. 4, reference numerals TA1, TA2, TA3 and TA4 are temperature distributions maintaining a temperature relationship of TA1>TA2>TA3>TA4.
That is, the temperature becomes high at portions TA1 where no part is mounted or where a semiconductor integrated circuit 211 having a small heat capacity is mounted, and the temperature becomes slightly low at portions TA2 where a mounting density of parts is high or parts having slightly large heat capacities are mounted. The temperature becomes further low at portions TA3 where a connector 212 having a relatively large heat capacity is provided, and becomes the lowest at portions TA4 where, for example, power source ferrite coils 213 are mounted.
As will be obvious from FIG. 4, a large proportion of the printed wiring board 201 is forming high-temperature portions (particularly, TA1) compared to the low-temperature portions (particularly, TA4).
According to the above method of setting the temperature of the low-temperature portions (usually, soldering portions) to the minimum temperature that maintains reliability while shielding the parts that are excessively heated, the high-temperature portions are wider than the low-temperature portions and, therefore, wide regions must be shielded. Therefore, problems arouse concerning not only the shielding but also elevating the heater temperatures and setting the temperature conditions to meet the overall increase in the heat capacities.
In view of the above problems inherent in the conventional reflow technologies, it is an object of the present invention to provide a reflowing apparatus which is capable of effecting the reflowing for the whole printed wiring board without causing parts to be heated in excess of their bearable temperatures by lowering the dispersion of temperature at the time of soldering that stems from the kinds and sizes of parts mounted on the printed wiring board and from the difference in the degree of packaging density, as well as to provide a reflowing method. In particular, it is the object of the invention to provide a reflowing apparatus adapted to using a lead-free solder which requires the treatment at a temperature higher than that of the ordinary solders and a reflowing method.